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Embedded Microstrip

Strictly speaking, the application of soldermask, conformal coating or other surface treatments will change a pure surface microstrip line to an ‘embedded or coated microstrip’ line. This, however, is not what is traditionally referred to as embedded microstrip line, and the determination of actual characteristic impedance is more difficult with these because of the inconsistency of dielectric constant and thickness of the material above the trace.

Microstrip is still by far the most commonly used transmission line structure in RF and microwave designs. This is becoming less the case though, as digital and mixed technology designs continue to increase in speed and density.

Typically, all connections in a digital circuit board are impedance controlled. This is by virtue of the fact that most layout designers will define, for example, 50 ohms as the default routing model for single ended connections.

Transmission Line Fundamentals

Over the last ten to twenty years, advances in IC fabrication technology have led to huge increases in both the operating frequencies and edge rates of switching signals in digital circuits. This has really pushed digital circuit design into the frequency domains that were previously considered RF and/or microwave. As a result, it has become important to consider the transmission line parameters of the connecting traces used for digital signals in circuit boards as much as they have always been considered for RF connections.

Today’s complex PCBs are being packed with a boatload of tiny components and are designed to run at ever-increasing speeds. So it certainly comes as no surprise to hear that silkscreen legends are fairly low on the designer’s priorities for consideration. However, the silkscreen’s function—that of aiding the manufacturer and engineer to locate components on the board—remains important.

Designers must be aware that the customer-supplied board outline shape isn’t always necessarily the shape that is to be delivered to the fabrication and assembly house for manufacturing. PCB’s must have two parallel sides (preferably the two longest sides) for processing them through the assembly equipment (conveyor transport).

In order to have a clean route, occasionally the designer may have to swap pins or gates on some devices. Usually the devices that require pin or gate swaps include resistor networks, capacitor networks, gated ICs (such as buffers), inverters, analog gates, and occasionally FPGAs.

Unless you are in an industry in which Printed Circuit Boards (PCB's) are used on a regular basis, you might not know exactly what they are. A PCB is an independent module of electronic interconnects. The green boards you see inside your electroncis are PCB's. Their function is to provide the electical connections between the electronic components mounted on them. PCB's are found in everyday devices such as TVs and more advanced things like radar systems. In this blog, we will cover the basice types of construction used for PCBs.

The silkscreen is a helpful tool in identifying components on the board. During PCB layout it should be designed for a consistent flow that allows easy identification of parts while using a minimum of leaders in difficult areas. Special care should be taken to ensure that each device has its appropriate associated markings, such as polarity, pin #1 location, 10th pin marker for fine pitch parts, or connector pinouts. Some parts may require additional labeling that should be specified by the customer. This may include labeling connectors, LEDs, switches, high voltage areas, or critical circuits (see the figure below).